In the past, construction machines such as a hydraulic excavator included an engine-driven hydraulic pump, which drove hydraulic work devices such as a bucket cylinder, an arm cylinder, a boom cylinder, and a travel hydraulic motor.
Further, the construction machines needed to have a high-power engine, which is less fuel efficient than a low-power engine, because it was necessary to significantly vary the power of the engine in order to handle a heavy load imposed during construction work.
However, high-power engines not only exhibit poor fuel efficiency but also cause environmental problems such as those related to exhaust gas and noise.
As such being the case, hybrid construction machines have been proposed in recent years to solve the above problems. These hybrid construction machines include a fuel-efficient low-power engine that generates an average power, and incorporate a technology for using electrical power stored in an electrical storage device to provide electric motor assist for the purpose of compensating for an power shortage caused by the use of the low-power engine.
Further, technologies for making efficient use of an electric motor have been devised. A conventional technology disclosed, for instance, in Patent Document 1 relates to a hybrid construction machine in which an electric motor and a generator are coupled to an engine to store electrical power remaining after light-load work in a battery and extract the electrical power stored in the battery for use during heavy-load work. If work load is not higher than a setting (is light) and the amount of electrical charge stored in an electrical storage device is not smaller than a setting (is large), this technology automatically stops the engine and switches to no engine operation in which the electric motor is driven by using only the electrical power stored in the electrical storage device. Further, if, during the no engine operation, the work load is not lower than the setting or the amount of electrical charge is not larger than the setting, this technology automatically restarts the engine. In other words, this technology makes it possible to keep the amount of electrical charge in the electrical storage device within a predetermined range by steadily operating the engine under light load conditions, that is, in a high-efficiency region, and providing increased energy savings by operating the electric motor to compensate for engine power shortage relative to the work load.
Meanwhile, a technology described in Patent Document 2 prevents an electrical storage device from being excessively charged by charging an electrical storage device with electrical power generated by a generator, which is rotationally driven by an engine, when the amount of electrical charge stored in the electrical storage device is insufficient, and by minimizing the power of the engine to an idling level or stopping the engine when the amount of electrical charge is sufficient.